Japanese Patent Application Publication No. 2003-254052 (JP-A-2003-254052) recites an exhaust purification system of an internal combustion engine. According to this system, the amount of poisoning substances adhering to an exhaust gas purification catalyst is estimated through calculations, and if the estimated adhering amount is larger than a reference amount, a poisoning curing process is executed in which the temperature of exhaust gas is forcibly increased by carrying out at least one of: reducing the air-fuel ratio (providing fuel-rich air-fuel mixtures); retarding the ignition timing; and suspending exhaust gas recirculation. Further, Japanese Patent Application Publication No. 08-189388 (JP-A-08-189388) recites an exhaust gas purification system that changes the temperature of an exhaust gas purification catalyst by making the air-fuel ratio of one or more of engine cylinders higher than the stoichiometric air-fuel ratio while making the air-fuel ratio of the rest of the engine cylinders lower than the stoichiometric air-fuel ratio. Further, Japanese Patent Application Publication No. 05-106498 (JP-A-05-106498) recites a control system that controls an internal combustion engine capable of running on alcohol-containing fuel. This control system detects the heaviness of fuel based on the alcohol concentration in the same fuel and controls the internal combustion engine based on the fuel heaviness and the alcohol concentration in the fuel.
The calorific value of alcohol (e.g., ethanol) per unit amount (will be simply referred to as “calorific value”) is smaller than that of hydrocarbon fuel (e.g., gasoline). Therefore, the calorific value of alcohol-containing fuel decreases as the alcohol concentration increases. Thus, in an internal combustion engine capable of running on alcohol-containing fuel, when a process for recovering an exhaust gas purification device (e.g., exhaust gas purification catalyst) is performed in which the exhaust gas purification device is heated by supplying unburned fuel to it by making the air-fuel ratio of one or more of the cylinders of the internal combustion engine lower (richer) than the stoichiometric air-fuel ratio, if the alcohol concentration of fuel is too high, the exhaust gas purification device may fail to be heated sufficiently. None of the technologies described in JP-A-2003-254052, JP-A-08-189388, and JP-A-05-106498 addresses the issue of the alcohol concentration in fuel at the time of performing the process for recovering the exhaust gas purification device, and, during the recovery process, the exhaust gas purification device may fail to be heated to a target temperature and thus the recovery process may fail to recover the exhaust gas purification device properly.